The present invention concerns a piezoelectric micro-resonator.
More precisely, the invention concerns a piezoelectric micro-resonator which vibrates in a main extension-compression mode and which is provided with a suspension assembly which is integral with the bar.
Piezoelectric micro-resonators in the form of bars are well known. They comprise a substantially parallelpipedal bar, for example of quartz, which has two faces, an upper face and a lower face, and two sides, and electrodes which are disposed on the bar. The electrodes are connected to an alternating current source and produce in the piezoelectric material, electrical fields the directions of which are fixed by the position of the electrodes. Moreover, it is well known that the electrical field produces stresses in the crystal, the direction of which depends on the direction of the electrical fields produced in the crystal with respect to the axis of the crystal. As is well known, a distinction is made for example in regard to quartz between the optical axis Z and systems of two orthogonal axis X and Y respectively an electric axis and a mechanical axis, which are contained in a plane perpendicular to the axis Z. A piezoelectric bar is said to be of Z cut when the upper and lower faces of the bar are substantially perpendicular to the optical axis Z. It will be appreciated that many other cuts are possible, but the invention is concerned with the case of piezoelectric bars with a cut in the vicinity of the Z cut.
Such piezoelectric micro-resonators are used in many situations, in particular for the production of time-keeping devices. Time-keeping devices of this kind are involved in the production of oscillator circuits forming the time bases for computers or electronic watches. In the case of watches, the resonators used are housed in sealed casings, the dimensions of which are to be as small as possible. The resonator itself must therefore be of very small dimensions. In addition, in watches, the only electrical power source is a battery which supplies in particular the resonator. In order to increase the service life of the battery, it is therefore necessary to use components which have the minimum possible level of power consumption. This obviously applies to the resonator, the energizing of which must require the minimum possible amount of electrical power. Now, the amount of power required increases in proportion to a decreasing quality factor. In addition, the electronic amplifier circuit required for energizing the oscillator must have a high output to convert the power supplied by the battery into a periodic signal in phase with the vibrations of the resonator. It is difficult to produce such a circuit, with a very low level of power consumption, in integrated form, when the piezoelectric coupling of the resonator is weak. The above-mentioned coupling may be measured by means of the dynamic capacity (C.sub.1) of the resonator. The value C.sub.1 for the Z cut bar operating in the extension-compression mode and provided with a system of electrodes according to the invention is given by the formula: ##EQU1## in which .epsilon..sub.r is the dielectric constant of the material used; L is the length of the electrode; w is the distance between the two sides of the bar (width) and t is the distance between the upper and lower faces (thickness).
A piezoelectric resonator in bar form provided with suspension arms, which vibrates in an extension-compression mode, is already known from Swiss Pat. No. 600.423. This resonator is shown in cross section and in simplified form in FIG. 1 of the accompanying drawings. The bar 2, the faces 2a and 2b of which are parallel to the plane XY is provided with two electrodes 4a and 6a on the edge of the upper face 2a and two electrodes 4b and 6b on the lower face 2b, with those electrodes being parallel to the Y direction. By setting the electrodes 4a and 4b at the same potential V.sub.1 and the electrodes 6a and 6b at another potential V.sub.2, an electrical field E is produced in the bar, which has a non-zero component on the X axis. This therefore provides a resonator which vibrates in extension-compression according to the Y axis. In addition, with the bar being perpendicular to the Z axis, it is easy for the bar to be cut with an outstanding degree of accuracy, by chemical attack. However, the excitation produced by means of the component of the electrical field which is parallel to the X axis is low. This means that the dynamic capacity of such a resonator is low, of the order of 0.1 fF, thus making it very difficult to produce a stable oscillator with a very low level of power consumption.
Also known are resonators which are produced by mechanical processes, having electrodes on the faces which are perpendicular to the X axis, to produce a sufficient piezoelectric coupling effect. However, these resonators do not have any fixing assembly which forms an integral part of the bar, so that they have to be mounted in the casing by means of conductor wires which are soldered to the faces of the resonator. Such a mode of assembly is obviously difficult and does not permit mass production.